Melt spun fabrics manufactured from synthetic thermoplastics have long been used in a variety of applications including filtration, batting, fabrics for oil cleanup, absorbents such as those used in diapers and feminine hygiene products, thermal insulation, and apparel and drapery for medical uses.
Melt spun materials fall in the general class of textiles referred to as nonwovens since they comprise randomly oriented filaments, or fibers, made by entangling the fibers through mechanical means. The fiber entanglement, with or without some interfiber fusion, imparts integrity and strength to the fabric. The nonwoven fabric may be converted to a variety of end use products as mentioned above.
Although melt spun nonwovens may be made by a number of processes, the most popular processes are meltblown and spunbond processes, both of which involve melt spinning of thermoplastic material. Meltblown is a process for the manufacture of a nonwoven fabric wherein a molten thermoplastic is extruded from a die tip to form a row of filaments. The fibers exiting the die tip are contacted with converging sheets or jets of hot air to stretch or draw the filaments down to microsize diameter. The fibers are then deposited onto a collector in a random manner and form a nonwoven fabric.
The spunbond process involves the extrusion of continuous filaments through a spinneret with multiple rows of filaments. The extruded filaments are maintained apart and the desired orientation of the filaments is achieved, for example, by electrical charges, by controlled air streams, or by the speed of the collector. The filaments are collected on the collector and bonded by passing the layer of filaments through compacting rolls and/or hot roll calendaring.
Nonwoven materials are used in such products as diapers, surgical gowns, carpet backings, filters and many other consumer and industrial products. The most popular machines for manufacturing nonwoven materials use meltblown and spunbond apparatus. For certain applications, it is desirable to utilize multiple types of thermoplastic liquid materials to form individual cross-sectional portions of each filament. Often, these multi-component filaments comprise two components and, therefore, are more specifically referred to as bicomponent filaments. For example, when manufacturing nonwoven materials for use in the garment industry, it may be desirable to produce bicomponent filaments having a sheath-core construction. The sheath may be formed from a softer material that is comfortable to the skin of an individual and the core may be formed from a stronger, but perhaps less comfortable material having greater tensile strength to provide durability to the fabric. Another important consideration involves the cost of the material. For example, a core of inexpensive material may be combined with a sheath of more expensive material. The core may be formed from polypropylene or nylon and the sheath may be formed from a polyester or co-polyester. Many other multi-component fiber configurations exist, including side-by-side, tipped, and microdenier configurations, each having its own special applications. Various material properties can be controlled using one or more of the component liquids. These include, as examples, thermal, chemical, electrical, optical, fragrance, and anti-microbial properties. Likewise, many types of die tips exist for combining the multiple liquid components just prior to discharge to produce filaments of the desired cross-sectional configuration.
Various apparatus form bi-component filaments with a die tip comprising vertically or horizontally stacked plates. In particular, a meltblown die tip directs two flows of liquid material to opposing sides near the top of a stack of vertical plates. A spunbond die tip directs two different material flows to the top plate of a stack of horizontal plates. Liquid passages etched or drilled into the vertical or horizontal stack of plates direct the two different types of liquid material to a location at which they are combined and extruded at the discharge outlets as multi-component filaments. Various cross-sectional configurations of filaments are achieved, such as side-by-side and sheath-core configuration.
Using a stack of thin plates in either a vertical or horizontal orientation manner suffers from imperfect seals between plates. In a production environment, liquid pressure will cause adjacent plates to move slightly away from each other. Thus, small amounts of liquid of one type can leak through these imperfect seals, causing “shot” or small balls of polymer to be formed in the extruded filaments. The shot causes the multi-component filaments to form with problems such as reduced strength or increased roughness. Also, the stacked plates may not offer a substantial thermal barrier between the two types of liquid material. Consequently, the filaments of each liquid material may not combine at their respective optimum temperatures, possibly adversely affecting extrusion thereof.
Other apparatus avoid the use of stacked plates by having the two types of liquid material combine in a cavity prior to extrusion of the two types of liquid through multiple discharge passages. More specifically, two different types of liquid materials, such as thermoplastic polymers, initially reside side-by-side in the cavity and are delivered under pressure to the discharge passages where they are extruded in side-by-side relation as bicomponent filaments. Since the two liquid materials reside in side-by-side relation in the die cavity and discharge passages, this may lead to thermal problems or problems related to the materials improperly combining or mixing prior to extrusion.
For these reasons, it is desirable to provide apparatus and methods for melt spinning multi-component filaments without encountering various problems of prior melt spinning apparatus.